The Orange Tuff is an informally named tephra-fall deposit that crops out extensively in the volc... more The Orange Tuff is an informally named tephra-fall deposit that crops out extensively in the volcanic highlands to the south and southwest of Bogor, West Java, Indonesia. The data included in this data release documents the deposit thickness and maximum sizes of lithics present at sites throughout its distribution. Additional tables document dispersal areas calculated from within hand-drawn isopach and lithic isopleth maps based on the aforementioned thickness and lithic size data. Such area measurements will be used in the related subsequent publication listed in the metadata to derive eruption and deposit characteristics such as bulk volume and column height and to help determine the source volcano. The isopach and isopleth maps will also be reported as figures in the subsequent publication. Additional tables contain geochemical data from electron microprobe analyses of constituent plagioclase, Fe-Ti oxides, and orthopyroxenes. All analyses were carried out at Nanyang Technologica...
agmatic monazites range from 293.1 ± 1.3 Ma (2σ) in the Carnmenellis pluton, to 274.5 ± 1.5 Ma in... more agmatic monazites range from 293.1 ± 1.3 Ma (2σ) in the Carnmenellis pluton, to 274.5 ± 1.5 Ma in the southern lobe of the Land's End pluton, and the smaller stocks were emplaced over a comparably extended period. Each pluton cooled through ca. 320°C (T c argon, muscovite) within 4�5 m.y. at the existing exposure level, but insignificant cooling has taken place since the Late Triassic at present depths of 2�2.5 km. Greisenbordered wolfra mite (± cassiterite) vein systems and lodes formed simultaneously with the cooling to ca. 320°C of the host intrusive rocks, lending support to an origin related to retrograde boiling of apical magmas. Major SnCu lode development in the Carn Brea district was initiated 1�3 m.y. after emplacement othe local Wrich veins, and was broadly coeval wi th cooling of the Carnmenellis pluton — through T c muscovite at 1�1.5 km below the greatest known dep ths of economic cassiterite deposition in the mineralized shearzones. Both clans of lithophilemetal d...
We estimated the power densities of 66 geothermal fields above 10 MWnet with more than 5 years of... more We estimated the power densities of 66 geothermal fields above 10 MWnet with more than 5 years of production history. Power density follows a log-normal probability distribution. The mean power density of the population is 15.4 MW/km 2 , the median is 12.0 MW/km 2 and the standard deviation is 9.5 MW/km 2 . Power density is a function of average reservoir temperature and correlates strongly with tectonic setting. Fault-based systems tend to be low to moderate-temperature and have low power density, volcanic arc systems tend to be moderate to high-temperature and have moderate to high power density, and rift systems tend to be high-temperature and have high power density. The one major deviation from these trends is that some high-temperature volcanic arc systems tend to have moderate to low power density which appears to be anti-correlated with temperature. One important reason for this may be that these volcanic arc-hosted systems tend to be found in more purely compressional setti...
In 2021, Philippine Geothermal Production Company, Inc. (PGPC) executed its deep drilling campaig... more In 2021, Philippine Geothermal Production Company, Inc. (PGPC) executed its deep drilling campaign in Mak-Ban which included nine deep (>3348mMD) production wells. The nine wells were designed to extract production solely from the deep reservoir. This strategy is in response to encroachment of shallow recharge which is known to have detrimental effects on the wells’ productivity. Permeability in the deep reservoir is postulated to be both structural and lithologic in nature. Olilia North Fault and Bulalo II Fault, the two faults considered as good permeability sources based on past drilling results, were identified as main structural targets. Contact zones of andesitic to basaltic rocks and silicic intrusives is another source of permeability in the deep reservoir. To confirm intersection with these targets, rock cuttings, drilling data, and gamma ray (GR) responses were evaluated. Silicic intrusives were indicated by the presence of high GR responses in the deep section of a well. Of the nine production wells, eight showed adequate high GR intervals in the deep reservoir. These wells targeted the northwest hotspot, central upflow and southeast upflow. Thicknesses of these high GR intervals range from tens to hundreds of meters. The thickest intervals were observed in wells drilled in the southeast. Petrographic analyses of the rock cuttings suggest that the high GR intervals are mostly silicic intrusives, but in the southeast, the thick silicic units are silicic ash-flow tuff and volcaniclastics. Evidence of structures appeared to be rare in rock cuttings and petrographic analyses. Thus, drilling data became the primary indicator. Several bit walk and drilling breaks were considered fault related. The differences in GR response and productivity between adjacent wells in the NW may also be a manifestation of a fault divide. The relationship between high GR intervals and permeable zones is being evaluated. One limitation of the analysis, however, is that permeable zones from completion tests are used instead of actual productive zones from flowing Pressure-Temperature-Spinner (PTS) surveys. Preliminary analysis shows that those wells targeting the southeast silicic unit have the strongest correlation between high GR intervals and their permeable zones. In the northwest, only one well exhibited a strong correlation between silicic units and permeable zones. The weakest connection was observed in the central sector. In terms of future targeting, this initial insight suggests that it may be most advantageous to target the southeast silicic units. Therefore, more effort will be exerted in modeling the silicic unit. Furthermore, the relationship between the southeast silicic unit and Bulalo II Fault needs to be investigated. Ultimately, to establish a more reliable correlation, productive zones interpreted from PTS surveys should be used.
The microscopic study of geothermal well samples (petrography) provides a window into rock compos... more The microscopic study of geothermal well samples (petrography) provides a window into rock composition, mineralogy, and texture that enhances understanding of both primary rock types and their hydrothermal alteration history. Integrating petrographic analysis with other data from wells in 3D geologic models reveals the underlying origins of permeability, its destruction and rejuvenation through time, and the present state of the system. Common assumptions about the correspondence of alteration assemblages and temperature do not always hold, and petrography often clarifies the reasons for such mismatches. Petrography also allows fine-tuning of stratigraphic and structural interpretations, identification of datable units, and clarification of key conceptual model elements. Examples of important and often neglected geologic details resolved with petrography include: 1) identification of sparse silicic rocks in dominantly andesitic systems that are good candidates for dating and sometimes have high permeability; 2) differentiation of intrusive rocks from thick lavas using textural and mineralogic criteria that help identify near-intrusive margins hosting permeable zones or pointing towards the system upflow location. Important mineralogic observations from petrography include: 1) epidote ± adularia ± quartz is commonly formed at fracture initiation in upflow zones; 2) prehnite, wairakite, and calcite are more commonly formed later and tend to result in permeability loss unless fractures are being reopened or new ones are being formed; 3) in some cases wairakite may be the most reliable indicator of reservoir top and extent; and 4) plagioclase is susceptible to dissolution and replacement by albite and adularia, commonly resulting in secondary microporosity, and movement of Ca +2 and Al +3 from regions of hot upflow to higher levels of the system. Detailed studies involving fluid inclusion analysis, SEM, cathodoluminescence, and age dating can be coordinated with petrography and provide more detailed confirmation of mineral compositions and system evolution.
This report presents preliminary 1:24,000-scale geologic maps of the eastern Big Horn and Belmont... more This report presents preliminary 1:24,000-scale geologic maps of the eastern Big Horn and Belmont Mountains in west-central Arizona. The mapping, completed between January and April 1985, was jointly funded by the U.S. Geological Survey and the Arizona Bureau of Geology and Mineral Technology as part of the cost-sharing, Cooperative Geologic Mapping Program (COGEOMAP). The aim of COGEOMAP is to produce high-quality geologic maps for areas that have been inadequately mapped. The Big Horn and Belmont Mountains were chosen because neither range had been previously mapped, except in broad reconnaissance for previous State geologic maps, and because both ranges have substantial mineralization and exploration activity. One report and two map sheets, 1:24,000 map scale.Documents in the AZGS Document Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact azgs-info@email.arizona.edu
The principal geologic feature of the Big Hom and Belmont Mountains is a complexly faulted and ti... more The principal geologic feature of the Big Hom and Belmont Mountains is a complexly faulted and tilted series of mostly Miocene volcanic rock that record a period of Middle Tertiary magmatism and extension. These volcanic rocks vary widely in composition, but basaltic and rhyolitic rocks are most abundant (Figure 1). Intrusive equivalents of these volcanics exposed in the Belmont Mountains are dominantly granitic. Despite the large volume of rhyolite erupted, small, coalescing flow and dome complexes were formed in preference to large-volume ash-flow tuffs, and no collapse calderas were formed. These rocks lie in the upper plate ofthe regional Whipple-Buckskin-Bullard detachment fault [Rehrig and Reynolds, 1980], at its southeastern tip [Richard et aI, 1990a]. A regional boundary between major tilts domains in Tertiary strata follows an irregular course from northwest to southeast through the range [Rehrig et aI., 1980]. Northeast of this boundary, strata dip to the northeast, and southwest of the boundary, Tertiary strata dip to the southwest. Pre-Tertiary rocks are exposed in several parts of the range. These rocks consist of a Laramide-age (71-63 Ma)hornblende-biotite granodiorite, informally referred to as the Big Hom granodiorite, which intrudes a complex of Proterozoic igneous and metamorphic rocks. Metamorphic grade in the Proterozoic rocks increases from southeast to northwest, and associated igneous rocks become more abundant to the northwest. In the southeast the Proterozoic rocks consist of highly deformed, greenschist facies, mafic and pelitic schist. These grade to heterogeneous mafic-tointermediate- composition gneiss to the northwest. Report with geologic map and cross sections of the Big Horn and Belmont Mountains, west-central Arizona. Scale 1:50,000.Documents in the AZGS Document Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact azgs-info@email.arizona.edu
The Awibengkok geothermal field, also known as Salak, is the largest developed geothermal resourc... more The Awibengkok geothermal field, also known as Salak, is the largest developed geothermal resource in Indonesia, currently sustaining 377 MW of electrical generation. It is a water-dominated, naturally fractured reservoir with benign fluid chemistry. A very large ...
Abstract-Homblende-dacitic pumice fragments (HBDP) and their disaggregation products (streaks, wi... more Abstract-Homblende-dacitic pumice fragments (HBDP) and their disaggregation products (streaks, wisps and xenocrysts) are common in the Tshirege (upper) Member of the Bandelier Tuff, generally comprising <l vol% of the unit. HBDP are typically ellipsoidal in shape and have a groundmass of acicular plagioclase and hornblende set in vesicular rhyolitic glass. Coarse-grained minerals were out of equilibrium in HBDP and represent either phenocrysts that grew prior to mixing (mainly plagioclase and pyroxene), or xenocrysts derived from entrained rhyolitic magma (dominantly alkali feldspar and quartz). Groundmass minerals (mainly plagioclase and hornblende)
mantle larger crystals and have textures and compositions that indicate rapid, undercooled crystallization from a limited supply of liquid. These features indicate that HBDP represent partially quenched blobs of a hybrid layer formed during addition of intermediate magma to the rhyolitic Bandelier chamber, and thus are equivalent to many quenched mafic inclusions in rhyolites and mafic microgranular enclaves in granites. Textural features of HBDP are due primarily to pre-eruptive processes, not vesiculation within the vent. HBDP are not cogenetic with the rhyolitic components of the Tshirege Member and their chemical variations are due mainly to crystal fractionation and mixing with rhyolitic magma. The presence of HBDP in the earliest erupted material suggests that addition of this magma may have destabilized the Bandelier system and triggered its eruption. This may have occurred as vesicular hybrid magma either plumed buoyantly through silicic magma, or moved more efficiently toward the vent and in the conduit system.
We review the key geologic elements of stratigraphy, structure, and hydrothermal alteration to be... more We review the key geologic elements of stratigraphy, structure, and hydrothermal alteration to better understand their roles in controlling the Muara Laboh geothermal system. The oldest rocks in the Muara Laboh region are the metamorphic Paleozoic Barisan Formation. Late Oligocene to Middle Miocene volcanism is represented by the Painan Formation, consisting of mixed volcanic and sedimentary rocks of andesitic to dacitic composition. In the Middle Miocene, granitic and granodioritic rocks intruded the Barisan and Painan Formations. Undifferentiated Silicic Volcanic and volcaniclastic rocks consisting dominantly of dacitic to rhyolitic tuff and sediments, are widely distributed northwest, west and southwest of Muara Laboh. Evidence from exploration wells indicates that this rock sequence is present mostly in the western Muara Laboh basin. In the eastern Muara Laboh Basin Andesitic Volcanics were erupted to the SE of Muara Laboh at about the same time. These sequences are overlain by ...
The 2014 ARGeo Program included gathering of new data and update of the conceptual model for the ... more The 2014 ARGeo Program included gathering of new data and update of the conceptual model for the Tendaho geothermal area, Ethiopia. The geothermal area is located within the NW-SE trending Tendaho Graben (TG), which includes the southern part of the younger and active Manda Hararo Rift (MHR) in the Afar region. Rifting and volcanic activity within TG occurred mostly between 1.8 and 0.6 Ma but extended to at least 0.2 Ma. Extension gave rise to about 1600 m of vertical displacement verified by drilling of the Afar Stratoid sequence, over a crust with a mean thickness of ~23 km. The main emphasis of our study was on the Dubti and Ayrobera geothermal areas, near the central axis of the TG, although we also reviewed data for Allalobeda, located near its western bounding faults. New data from these areas include magnetotelluric (MT) and time domain (TDEM) surveys completed in 2013-2014, as well as radon and shallow temperature surveys. The digital enhancement and modeling of existing pot...
Journal of Volcanology and Geothermal Research, 2001
Chemical analyses of 30 melt inclusions from Satsuma-Iwojima volcano, Japan, were carried out to ... more Chemical analyses of 30 melt inclusions from Satsuma-Iwojima volcano, Japan, were carried out to investigate volatile evolution in a magma chamber beneath the volcano from about 6300yr BP to the present. Large variations in volatile concentrations of melts were ...
The Orange Tuff is an informally named tephra-fall deposit that crops out extensively in the volc... more The Orange Tuff is an informally named tephra-fall deposit that crops out extensively in the volcanic highlands to the south and southwest of Bogor, West Java, Indonesia. The data included in this data release documents the deposit thickness and maximum sizes of lithics present at sites throughout its distribution. Additional tables document dispersal areas calculated from within hand-drawn isopach and lithic isopleth maps based on the aforementioned thickness and lithic size data. Such area measurements will be used in the related subsequent publication listed in the metadata to derive eruption and deposit characteristics such as bulk volume and column height and to help determine the source volcano. The isopach and isopleth maps will also be reported as figures in the subsequent publication. Additional tables contain geochemical data from electron microprobe analyses of constituent plagioclase, Fe-Ti oxides, and orthopyroxenes. All analyses were carried out at Nanyang Technologica...
agmatic monazites range from 293.1 ± 1.3 Ma (2σ) in the Carnmenellis pluton, to 274.5 ± 1.5 Ma in... more agmatic monazites range from 293.1 ± 1.3 Ma (2σ) in the Carnmenellis pluton, to 274.5 ± 1.5 Ma in the southern lobe of the Land's End pluton, and the smaller stocks were emplaced over a comparably extended period. Each pluton cooled through ca. 320°C (T c argon, muscovite) within 4�5 m.y. at the existing exposure level, but insignificant cooling has taken place since the Late Triassic at present depths of 2�2.5 km. Greisenbordered wolfra mite (± cassiterite) vein systems and lodes formed simultaneously with the cooling to ca. 320°C of the host intrusive rocks, lending support to an origin related to retrograde boiling of apical magmas. Major SnCu lode development in the Carn Brea district was initiated 1�3 m.y. after emplacement othe local Wrich veins, and was broadly coeval wi th cooling of the Carnmenellis pluton — through T c muscovite at 1�1.5 km below the greatest known dep ths of economic cassiterite deposition in the mineralized shearzones. Both clans of lithophilemetal d...
We estimated the power densities of 66 geothermal fields above 10 MWnet with more than 5 years of... more We estimated the power densities of 66 geothermal fields above 10 MWnet with more than 5 years of production history. Power density follows a log-normal probability distribution. The mean power density of the population is 15.4 MW/km 2 , the median is 12.0 MW/km 2 and the standard deviation is 9.5 MW/km 2 . Power density is a function of average reservoir temperature and correlates strongly with tectonic setting. Fault-based systems tend to be low to moderate-temperature and have low power density, volcanic arc systems tend to be moderate to high-temperature and have moderate to high power density, and rift systems tend to be high-temperature and have high power density. The one major deviation from these trends is that some high-temperature volcanic arc systems tend to have moderate to low power density which appears to be anti-correlated with temperature. One important reason for this may be that these volcanic arc-hosted systems tend to be found in more purely compressional setti...
In 2021, Philippine Geothermal Production Company, Inc. (PGPC) executed its deep drilling campaig... more In 2021, Philippine Geothermal Production Company, Inc. (PGPC) executed its deep drilling campaign in Mak-Ban which included nine deep (>3348mMD) production wells. The nine wells were designed to extract production solely from the deep reservoir. This strategy is in response to encroachment of shallow recharge which is known to have detrimental effects on the wells’ productivity. Permeability in the deep reservoir is postulated to be both structural and lithologic in nature. Olilia North Fault and Bulalo II Fault, the two faults considered as good permeability sources based on past drilling results, were identified as main structural targets. Contact zones of andesitic to basaltic rocks and silicic intrusives is another source of permeability in the deep reservoir. To confirm intersection with these targets, rock cuttings, drilling data, and gamma ray (GR) responses were evaluated. Silicic intrusives were indicated by the presence of high GR responses in the deep section of a well. Of the nine production wells, eight showed adequate high GR intervals in the deep reservoir. These wells targeted the northwest hotspot, central upflow and southeast upflow. Thicknesses of these high GR intervals range from tens to hundreds of meters. The thickest intervals were observed in wells drilled in the southeast. Petrographic analyses of the rock cuttings suggest that the high GR intervals are mostly silicic intrusives, but in the southeast, the thick silicic units are silicic ash-flow tuff and volcaniclastics. Evidence of structures appeared to be rare in rock cuttings and petrographic analyses. Thus, drilling data became the primary indicator. Several bit walk and drilling breaks were considered fault related. The differences in GR response and productivity between adjacent wells in the NW may also be a manifestation of a fault divide. The relationship between high GR intervals and permeable zones is being evaluated. One limitation of the analysis, however, is that permeable zones from completion tests are used instead of actual productive zones from flowing Pressure-Temperature-Spinner (PTS) surveys. Preliminary analysis shows that those wells targeting the southeast silicic unit have the strongest correlation between high GR intervals and their permeable zones. In the northwest, only one well exhibited a strong correlation between silicic units and permeable zones. The weakest connection was observed in the central sector. In terms of future targeting, this initial insight suggests that it may be most advantageous to target the southeast silicic units. Therefore, more effort will be exerted in modeling the silicic unit. Furthermore, the relationship between the southeast silicic unit and Bulalo II Fault needs to be investigated. Ultimately, to establish a more reliable correlation, productive zones interpreted from PTS surveys should be used.
The microscopic study of geothermal well samples (petrography) provides a window into rock compos... more The microscopic study of geothermal well samples (petrography) provides a window into rock composition, mineralogy, and texture that enhances understanding of both primary rock types and their hydrothermal alteration history. Integrating petrographic analysis with other data from wells in 3D geologic models reveals the underlying origins of permeability, its destruction and rejuvenation through time, and the present state of the system. Common assumptions about the correspondence of alteration assemblages and temperature do not always hold, and petrography often clarifies the reasons for such mismatches. Petrography also allows fine-tuning of stratigraphic and structural interpretations, identification of datable units, and clarification of key conceptual model elements. Examples of important and often neglected geologic details resolved with petrography include: 1) identification of sparse silicic rocks in dominantly andesitic systems that are good candidates for dating and sometimes have high permeability; 2) differentiation of intrusive rocks from thick lavas using textural and mineralogic criteria that help identify near-intrusive margins hosting permeable zones or pointing towards the system upflow location. Important mineralogic observations from petrography include: 1) epidote ± adularia ± quartz is commonly formed at fracture initiation in upflow zones; 2) prehnite, wairakite, and calcite are more commonly formed later and tend to result in permeability loss unless fractures are being reopened or new ones are being formed; 3) in some cases wairakite may be the most reliable indicator of reservoir top and extent; and 4) plagioclase is susceptible to dissolution and replacement by albite and adularia, commonly resulting in secondary microporosity, and movement of Ca +2 and Al +3 from regions of hot upflow to higher levels of the system. Detailed studies involving fluid inclusion analysis, SEM, cathodoluminescence, and age dating can be coordinated with petrography and provide more detailed confirmation of mineral compositions and system evolution.
This report presents preliminary 1:24,000-scale geologic maps of the eastern Big Horn and Belmont... more This report presents preliminary 1:24,000-scale geologic maps of the eastern Big Horn and Belmont Mountains in west-central Arizona. The mapping, completed between January and April 1985, was jointly funded by the U.S. Geological Survey and the Arizona Bureau of Geology and Mineral Technology as part of the cost-sharing, Cooperative Geologic Mapping Program (COGEOMAP). The aim of COGEOMAP is to produce high-quality geologic maps for areas that have been inadequately mapped. The Big Horn and Belmont Mountains were chosen because neither range had been previously mapped, except in broad reconnaissance for previous State geologic maps, and because both ranges have substantial mineralization and exploration activity. One report and two map sheets, 1:24,000 map scale.Documents in the AZGS Document Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact azgs-info@email.arizona.edu
The principal geologic feature of the Big Hom and Belmont Mountains is a complexly faulted and ti... more The principal geologic feature of the Big Hom and Belmont Mountains is a complexly faulted and tilted series of mostly Miocene volcanic rock that record a period of Middle Tertiary magmatism and extension. These volcanic rocks vary widely in composition, but basaltic and rhyolitic rocks are most abundant (Figure 1). Intrusive equivalents of these volcanics exposed in the Belmont Mountains are dominantly granitic. Despite the large volume of rhyolite erupted, small, coalescing flow and dome complexes were formed in preference to large-volume ash-flow tuffs, and no collapse calderas were formed. These rocks lie in the upper plate ofthe regional Whipple-Buckskin-Bullard detachment fault [Rehrig and Reynolds, 1980], at its southeastern tip [Richard et aI, 1990a]. A regional boundary between major tilts domains in Tertiary strata follows an irregular course from northwest to southeast through the range [Rehrig et aI., 1980]. Northeast of this boundary, strata dip to the northeast, and southwest of the boundary, Tertiary strata dip to the southwest. Pre-Tertiary rocks are exposed in several parts of the range. These rocks consist of a Laramide-age (71-63 Ma)hornblende-biotite granodiorite, informally referred to as the Big Hom granodiorite, which intrudes a complex of Proterozoic igneous and metamorphic rocks. Metamorphic grade in the Proterozoic rocks increases from southeast to northwest, and associated igneous rocks become more abundant to the northwest. In the southeast the Proterozoic rocks consist of highly deformed, greenschist facies, mafic and pelitic schist. These grade to heterogeneous mafic-tointermediate- composition gneiss to the northwest. Report with geologic map and cross sections of the Big Horn and Belmont Mountains, west-central Arizona. Scale 1:50,000.Documents in the AZGS Document Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact azgs-info@email.arizona.edu
The Awibengkok geothermal field, also known as Salak, is the largest developed geothermal resourc... more The Awibengkok geothermal field, also known as Salak, is the largest developed geothermal resource in Indonesia, currently sustaining 377 MW of electrical generation. It is a water-dominated, naturally fractured reservoir with benign fluid chemistry. A very large ...
Abstract-Homblende-dacitic pumice fragments (HBDP) and their disaggregation products (streaks, wi... more Abstract-Homblende-dacitic pumice fragments (HBDP) and their disaggregation products (streaks, wisps and xenocrysts) are common in the Tshirege (upper) Member of the Bandelier Tuff, generally comprising <l vol% of the unit. HBDP are typically ellipsoidal in shape and have a groundmass of acicular plagioclase and hornblende set in vesicular rhyolitic glass. Coarse-grained minerals were out of equilibrium in HBDP and represent either phenocrysts that grew prior to mixing (mainly plagioclase and pyroxene), or xenocrysts derived from entrained rhyolitic magma (dominantly alkali feldspar and quartz). Groundmass minerals (mainly plagioclase and hornblende)
mantle larger crystals and have textures and compositions that indicate rapid, undercooled crystallization from a limited supply of liquid. These features indicate that HBDP represent partially quenched blobs of a hybrid layer formed during addition of intermediate magma to the rhyolitic Bandelier chamber, and thus are equivalent to many quenched mafic inclusions in rhyolites and mafic microgranular enclaves in granites. Textural features of HBDP are due primarily to pre-eruptive processes, not vesiculation within the vent. HBDP are not cogenetic with the rhyolitic components of the Tshirege Member and their chemical variations are due mainly to crystal fractionation and mixing with rhyolitic magma. The presence of HBDP in the earliest erupted material suggests that addition of this magma may have destabilized the Bandelier system and triggered its eruption. This may have occurred as vesicular hybrid magma either plumed buoyantly through silicic magma, or moved more efficiently toward the vent and in the conduit system.
We review the key geologic elements of stratigraphy, structure, and hydrothermal alteration to be... more We review the key geologic elements of stratigraphy, structure, and hydrothermal alteration to better understand their roles in controlling the Muara Laboh geothermal system. The oldest rocks in the Muara Laboh region are the metamorphic Paleozoic Barisan Formation. Late Oligocene to Middle Miocene volcanism is represented by the Painan Formation, consisting of mixed volcanic and sedimentary rocks of andesitic to dacitic composition. In the Middle Miocene, granitic and granodioritic rocks intruded the Barisan and Painan Formations. Undifferentiated Silicic Volcanic and volcaniclastic rocks consisting dominantly of dacitic to rhyolitic tuff and sediments, are widely distributed northwest, west and southwest of Muara Laboh. Evidence from exploration wells indicates that this rock sequence is present mostly in the western Muara Laboh basin. In the eastern Muara Laboh Basin Andesitic Volcanics were erupted to the SE of Muara Laboh at about the same time. These sequences are overlain by ...
The 2014 ARGeo Program included gathering of new data and update of the conceptual model for the ... more The 2014 ARGeo Program included gathering of new data and update of the conceptual model for the Tendaho geothermal area, Ethiopia. The geothermal area is located within the NW-SE trending Tendaho Graben (TG), which includes the southern part of the younger and active Manda Hararo Rift (MHR) in the Afar region. Rifting and volcanic activity within TG occurred mostly between 1.8 and 0.6 Ma but extended to at least 0.2 Ma. Extension gave rise to about 1600 m of vertical displacement verified by drilling of the Afar Stratoid sequence, over a crust with a mean thickness of ~23 km. The main emphasis of our study was on the Dubti and Ayrobera geothermal areas, near the central axis of the TG, although we also reviewed data for Allalobeda, located near its western bounding faults. New data from these areas include magnetotelluric (MT) and time domain (TDEM) surveys completed in 2013-2014, as well as radon and shallow temperature surveys. The digital enhancement and modeling of existing pot...
Journal of Volcanology and Geothermal Research, 2001
Chemical analyses of 30 melt inclusions from Satsuma-Iwojima volcano, Japan, were carried out to ... more Chemical analyses of 30 melt inclusions from Satsuma-Iwojima volcano, Japan, were carried out to investigate volatile evolution in a magma chamber beneath the volcano from about 6300yr BP to the present. Large variations in volatile concentrations of melts were ...
Uploads
mantle larger crystals and have textures and compositions that indicate rapid, undercooled crystallization from a limited supply of liquid. These features indicate that HBDP represent partially quenched blobs of a hybrid layer formed during addition of intermediate magma to the rhyolitic Bandelier chamber, and thus are equivalent to many quenched mafic inclusions in rhyolites and mafic microgranular enclaves in granites. Textural features of HBDP are due primarily to pre-eruptive processes, not vesiculation within the vent. HBDP are not cogenetic with the rhyolitic components of the Tshirege Member and their chemical variations are due mainly to crystal fractionation and mixing with rhyolitic magma. The presence of HBDP in the earliest erupted material suggests that addition of this magma may have destabilized the Bandelier system and triggered its eruption. This may have occurred as vesicular hybrid magma either plumed buoyantly through silicic magma, or moved more efficiently toward the vent and in the conduit system.
mantle larger crystals and have textures and compositions that indicate rapid, undercooled crystallization from a limited supply of liquid. These features indicate that HBDP represent partially quenched blobs of a hybrid layer formed during addition of intermediate magma to the rhyolitic Bandelier chamber, and thus are equivalent to many quenched mafic inclusions in rhyolites and mafic microgranular enclaves in granites. Textural features of HBDP are due primarily to pre-eruptive processes, not vesiculation within the vent. HBDP are not cogenetic with the rhyolitic components of the Tshirege Member and their chemical variations are due mainly to crystal fractionation and mixing with rhyolitic magma. The presence of HBDP in the earliest erupted material suggests that addition of this magma may have destabilized the Bandelier system and triggered its eruption. This may have occurred as vesicular hybrid magma either plumed buoyantly through silicic magma, or moved more efficiently toward the vent and in the conduit system.